Composite materials are of great current interest because they provide a very favorable combination of high strength and low density. Typically, a composite material is comprised of fibers of graphite, boron, glass, and the like embedded within an epoxy, phenolic or other polymer resin matrix. The more advanced composites which have particularly favorable high strength to density ratio properties are especially attractive for aerospace applications. But typical of other advanced aerospace materials they present comparative processing difficulties; they cannot be made by a simple layup of the fibers and resin followed by room temperature curing. Aerospace composite materials not only involve more difficult-to-fabricate resins but often essentially defect-free finished parts must be produced. As a result, aerospace composites are typically molded and cured at elevated temperatures under substantial pressure.
A desired molding cycle can be obtained by compression molding, whereby composite prepreg materials are placed between the heated platens of a unidirectionally-acting press. But, metal compression molding dies can be expensive for complicated shapes. Lower cost rubber compression dies tend to be thick and insulative, slowing cure time. More importantly, with either type of die, complex surfaces will not be subjected to uniform pressure.
To overcome these limitations, parts are molded while being subjected to isostatic pressure. In a widely used procedure, the prepreg for the article is placed in an evacuated impervious flexible bag and subjected to simultaneous heating and isostatic pressure from a gas or a liquid. To give shape to the composite article the prepreg is often adhered to a rigid structure, such as a metal piece. Problems are connected with this process: any leak in the evacuated bag either lessens the requisite pressure on the article or allows interaction between the pressurizing medium and the partially cured polymer. In fact, such leaks are not uncommon and the resultant rejection rate is significant, especially at higher molding temperatures and pressures.
To overcome some of the aforementioned limitations, use has been made of pressure pad molding. See the article "Silicone Rubber Puts on the Squeeze for High Quality Composite Layups", Plastics World, June 16, 1975 (Cahners Publishing Co., Inc., Boston, Mass.). In this process, use is made of shaped pads of a high thermal expansion silicone rubber, such as the Silastic J type tooling rubber of Dow Corning Corporation, Midland, Mich. USA. The uncured prepreg is contained within a space between abutting adjacent pads and the assembly is captured in a closely fitting closed metal vessel. The vessel and contained assembly are then heated to an elevated temperature to both cure the article and expand the rubber. Since a typical silicone rubber has a thermal expansion coefficient of about 18 times higher than that of both a typical steel vessel and the typical composite article material, upon heating the expanding trapped rubber subjects the composite material to a substantial pressure, thus desirably forming the part.
The pressure pad molding technique is advantageous in that it can overcome the leakage problems of isostatic pressing. Any bag leak does not adversely affect the molding pressure or cause interaction. However, a problem with the pressure pad method is the interdependency of temperature and pressure; many desired temperature-pressure cycles are not obtainable (e.g., sustaining pressure on cooling). Another problem is that the temperature-pressure cycle is a function of the mechanical fit between the various system components; a chosen cycle will vary when there is a small change in the dimensions of the several components. For example, at room temperature the pad usually is volumetrically 6%-8% smaller than the metal vessel interior, to avoid excessive peak pressures. But, at peak temperature, a change in this dimension from about 7% to 8.2% in a typical situation will change the maximum pressure from about 7.2 MPa to 1.7 MPa. Consequently, when a pressure pad is replaced with a new pad not having precisely the same dimensions, or when there is some permanent set of the rubber with use, a different temperature-pressure cycle will be undesirably produced. In many of the more advanced composite systems, the foregoing are significant disadvantages of the pressure pad method.
Isostatic pressure vessels for molding are also widely used. Typically, the vessels are strongly made, as shown by U.S. Pat. No. 3,419,935 to Pfeiler et al. As a general proposition, various gases from an external source are used to apply the molding pressure to the article which is contained in an evacuated bag. While a gaseous medium is typically characterized by a relatively low thermal conductivity, convective heat transfer can ordinarily cause temperature variations as referred to in the Pfeiler patent. In the liquid isostatic pressing method, a liquid medium such as water is used to apply pressure to the article in the same manner as employed with gas. The heat transfer phenomena are similar.
Isostatic pressing has been widely used in the powder metal and ceramics field as well as in the polymer field. See for instance, U.S. Pat. No. 3,462,797 to Asbury and U.S. Pat. No. 3,279,917 to Ballard et al. There is a great variety in the design of isostatic pressing devices insofar as heating techniques, but as a general proposition, heating of the walls of the higher operating temperature systems is avoided.
The pressure pad technique described above has been used to mold polymers in particular because it offers improvements over fluid isostatic pressing. Pressure pad molding uses a pressure vessel but the pressure vessel need not be gas or liquid tight; the vessel is nearly filled with a silicone tooling rubber having within it a cavity in which the article being molded is placed. The cavity shapes the part and it is for this reason that tooling rubber is used. Tooling rubber is a filled elastomer having comparatively good strength and resistance to abrasion and deformation. Upon heating of the vessel, the rubber and article contained therein, the differential expansion between the high coefficient of expansion rubber and the low coefficient of expansion steel causes the rubber to seek to expand beyond the confines of the vessel, thereby increasing the pressure on the article contained therein. However, because of the strength characteristics of the tooling rubber, this method does not tend to produce an isostatic force on the article. Rather, uneven force is produced according to the local variance in fit and shape between the vessel, rubber piece, and article precursor.
U.S. Pat. No. 4,264,556 to Kumar et al "Thermal Isostatic Densifying Method and Apparatus" describes a special isostatic pressing vessel and process wherein a medium such as water or liquid bismuth may be placed in a vessel surrounding the article precursor, and the pressure is varied by adding or extracting thermal energy from the fluid medium to change its volume or its state from solid.
As mentioned above, when gases and liquids are used to apply pressure to an article, obvious problems arise when there is leakage of the bag or closure in which the article precursor is contained. If there is leakage in the bag, the pressure applied to the article precursor is lost. And even if very small leaks are compensated for by continuous evacuation of the article precursor bag there can be pressure gradients or chemical interaction with the material being molded. In the pressure pad molding method, there has been employed general heating of the entire vessel and contents, and this is often inconvenient. The pressure pad molding method apparatus does not tend to produce uniform or isostatic forces on a article precursor, as mentioned above. Fit of the rubber in the vessel is critical in limiting peak pressure. And most importantly, there is no possible independent control of pressure other than by means of the average temperature of the rubber. This leads to limitation on choice of cycle, especially on cooldown.
Accordingly, what is needed in this art is a method and apparatus which overcomes these problems of the prior art.